1. Field of the Invention
The present invention relates to an inertial drive actuator.
2. Description of the Related Art
When a drive pulse of a waveform formed of a gently rising part, and a rapidly falling part in continuity with the gently rising portion is applied to an electromechanical transducer such as a piezoelectric element, in the gently rising part of the drive pulse, the piezoelectric element is displaced by being elongated gently in a direction of thickness, and in the rapidly falling part, the piezoelectric element is displaced by being contracted rapidly. Given this, an actuator in which, using this characteristic, by applying a drive pulse of a waveform as mentioned above to the piezoelectric element, charging and discharging are repeated at different speeds, and vibrations are generated in a direction of thickness in the piezoelectric element, at different speeds, and a drive shaft fixed to the piezoelectric element is let to make reciprocating movement at different speeds, and a mobile object which is attached to the driving member is moved in a predetermined direction has hitherto been known (for example, refer to Japanese Patent Application Laid-open Publication No. 2003-185406)
As a conventional example of such actuator, an actuator 200 shown in FIG. 13A and FIG. 13B is available. FIG. 13A is a perspective view showing a structure of the conventional actuator 200, and FIG. 13B is a cross-sectional view taken along a line XIIIB-XIIIB in FIG. 13A. The actuator 200 acquires a frictional force by a mobile object 204 being pressed by a drive shaft 203. A method in which, a plate spring is used has been widely used as a method for acquiring the frictional force. Here, the frictional force is imparted by inserting a pinching member 205, and pressing the pinching member 205 from above by an elastic member 206. The pinching member 205 is fitted tightly to the mobile object 204 in a direction of movement of the drive shaft 203. With the abovementioned structure, a pressing force generated by the elastic member 206 is transmitted to the drive shaft 203 via the pinching member 205. Therefore, even when the drive shaft 203 changes at a different speed in a positive direction and a negative direction in an axial direction, the elastic member 206 does not undergo an elastic deformation, and it is possible to drive the mobile object 204 stably at a high speed.
Moreover, an actuator shown in FIG. 14 can be cited as a conventional actuator. FIG. 14 is a schematic diagram showing a structure of a conventional actuator 300. The actuator 300 shown in FIG. 14 is capable of detecting a position, and includes a piezoelectric element 301, a drive shaft 302, a mobile object 303 attached to the drive shaft 302, a frame 304 of the actuator, and a detecting member 305. One end of the piezoelectric element 301 is fixed by adhering to the frame 304, and the other end of the piezoelectric element 301 is fixed by adhering to the drive shaft 302. The mobile object 303 is slidably movable on the drive shaft 302.
The detecting member 305 forms a fixed electrode for detecting a position of the mobile object 303, based on an electrostatic capacitance, which is arranged not to be in contact, parallel along a direction of movement of the mobile object 303, and is fixed to the frame 304. Moreover, the drive shaft 302, the mobile object 303, and the detecting member 305 are formed of an electroconductive material.
On a surface of the detecting member 305 facing the mobile object 303, recesses and projections are formed at a fixed interval along the direction of movement of the mobile object 303, thereby forming an electrode 305a. The electrode 305a and the mobile object 303 are positioned face-to-face, to be isolated at a distance D, and form a condenser of electrostatic capacitance C. Since an electrostatic capacitance between the mobile object 303 and the projection on the electrode 305a is higher than an electrostatic capacitance between the mobile object 303 and the recess on the electrode 305a, by moving of the mobile object 303, the electrostatic capacitance between the mobile object 303 and the electrode 305a changes periodically. The actuator 300 detects a position of the mobile object 303 by capturing a cycle of such change of electrostatic capacitance.
However, when an actuator section 310 including the piezoelectric element 301, the drive shaft 302, and the mobile object 303, and a position sensor section 320 including the detecting member 305 are installed separately, since each of the actuator section 310 and the position sensor section 320 has a mobile shaft and a detection shaft, the two shafts may be misaligned according to a method of installing. In a position sensor which detects the electrostatic capacitance of the actuator 300 in the conventional example, a value of the electrostatic capacitance changes according to a gap between the mobile object 303 and the electrode 305a. Therefore, when the mobile shaft and the detection shaft are misaligned in a direction of gap, the gap between the mobile object 303 and the electrode 305a changes according to the position of the mobile object 303. Consequently, the electrostatic capacitance changes according to the position of the mobile object 303, thereby causing a decline in accuracy of detection, and it becomes difficult to detect the position accurately. Moreover, it is extremely difficult to dispose the mobile object 303 and the electrode 305a to be mutually parallel in order to maintain the gap to be same over the entire range of movement of the mobile object 303, and to improve the position accuracy, an assembling accuracy is to be improved.